This is an interdisciplinary research project between the departments of Neuroradiology and Neurology at Columbia University. It is proposed to study the neurotoxic effect of myelographic water-soluble contrast media using electrically evoked neural activity in the central nervous system. With this basic model, a great insight has been obtained on the effect of ionic and non-ionic media on the spinal cord. The effect on mono- and polysynaptic responses was extensively researched. We have also developed, with the cooperation of one manufacturer, a new water-soluble radiopaque medium that appears to be ten times less neurotoxic than metrizamide as judged by its convulsive effect in the non-anesthesized Rhesus monkey. The result was achieved by substituting a slight change on the tri-iodo benzene ring with one that is more hydrophilic.